1. Field
This invention relates generally to a flared antenna feed horn and, more particularly, to a flared antenna feed horn that includes a flared outer conductor, a microstrip-to-coaxial transition TE11 sum mode launcher and a TE12 difference mode launcher.
2. Discussion
For some communications applications, it is desirable to have a broadband system, namely, operation over a relatively wide frequency range, typically greater than 1.5:1. In some reflector based systems, it is desirable to have a feed with a small foot print, making it suitable for illuminating very low focal length to diameter ratio reflector lens.
In certain communications systems, signal tracking between the receiver and transmitter is achieved using a sum and difference radiation pattern. A sum pattern provides a broadside peak radiation pattern and a difference pattern provides a broadside null radiation pattern. In this case, two electromagnetic propagation modes, particularly the transverse-electric (TE) modes TE11 and TE12, are needed to realize a sum and difference within the same frequency range. System performance requirements may include a large instantaneous RF bandwidth and a small physical footprint, as well as other requirements.
A critical element to achieve the signal tracking feature, while meeting system specifications is the feed antenna. To meet desired size constraints, a smaller aperture size is usually required, such as that of an antenna feed horn. However, the cut-off frequency of the TE12 difference mode of an antenna feed horn is about twice the cut-off frequency of the TE11 sum mode, where the cut-off frequency of a particular mode is the lowest frequency that the mode can propagate. It is known in the art to load such a feed horn with a dielectric to lower the cut-off frequency of a particular mode. In addition to realizing the necessary modes for generating the sum and difference modes, ample signal from the feed horn must be transmitted or received. Namely, for a small aperture relative to the operating wavelength feed horn, there exists a significant impedance mismatch between the dielectric and free space resulting in significant signal loss.